As far as I know, RAID-6 has an even worse penalty on writes than its cousin RAID-5.
But we also know that we can get more performance out of a typical RAID array by having more spindles in it. But how much more?
Typical RAID5 arrays will have between 5 and 8 total disks. If we want to match the same performance (more in terms of IOPS rather than bandwidth, and available space is irrelevant) with a RAID6 array how many disks should we put in it? one more? two more? 30% more?
EDIT: of course I'm talking about using homogeneous disks (same capacity & performance)
2nd EDIT: I'm talking about rules of thumbs and not specific numbers. There must be some rough numbers about this...